1. Field of the Invention
The present invention relates to an image display apparatus such as a monitor or a television, and more particularly, to a method of improving the gradation of an image and an image display apparatus for performing the method.
2. Description of the Related Art
The gradation of an image in the image display apparatus is one of the factors that determine the quality of the image. Unlike a cathode-ray tube (CRT) adopting an electron gun, the performance of a general liquid crystal display (LCD) or liquid-crystal-on-silicon (LCoS) display, which uses liquid crystal, may abruptly change according to the physical characteristics of the crystal used or a method of driving the liquid crystal. Here, the performance is related to the factor with which is transmitted to or reflected from a liquid crystal display panel according to liquid crystal driving voltage. As a result, a general LCD is not capable of appropriately displaying an image having more than a predetermined number of gradations, e.g., 8-bit (28) gradations, on each of R, G, and B channels (here R, G, and B denote ‘red’, ‘green’ and ‘blue, respectively). Even if the 8-bit gradations are all displayed, an irregular difference in the luminance levels of the gradations cannot be removed. Therefore, in a general LCD, when the number of gradations is insufficient or a difference between luminance levels among gradations of the image is irregular, rough gradation borders are prone to occur at an image of a face at which gradations change gradually.
Hereinafter, conventional methods of improving the gradation of an image will be described.
First, when the number of gradations displayed is insufficient, the number of insufficient gradations is increased spatially or using time division. In particular, a half-toning method is commonly used to increase the number of gradation spatially. The half-toning method is subdivided into a dithering method of displaying medium gradations using pixels of predetermined area, e.g., 3×3, and an error diffusion method of comparing an input value of each pixel with values capable of being output and then diffusing a difference between an input value and the output value, i.e., an error value, to neighboring pixels. Here, one of dithering methods is disclosed in U.S. Pat. No. 3,937,878 entitled “Animated Dithered Display Systems”. In the disclosed dithering method, gradations are displayed with an area mask, and thus, regions of an image having high frequency components are difficult to be displayed, i.e. the resolution of an image may deteriorate. One of error diffusion methods is disclosed in U.S. Pat. No. 5,162,925 entitled “Color Image Processor Capable of Performing Masking Using a Reduced Number of Bits”. The disclosed error diffusion method overcomes deterioration of the resolution of an image caused by the dithering method, but additionally requires a frame memory of predetermined size to calculate diffused errors, thereby making the structure of a system complex and voluminous. Further, the disclosed error diffusion method generates peculiar patterns at the edge of an image or a color-flattened region.
A method of controlling frame-rate is a typical method of increasing the number of insufficient gradations using time division. In this method, a unit image frame is divided into sub-frames having different periods of emitting light, e.g., eight sub-frames, on a time axis, and then, these sub-frames are combined to display the gradations of an image. This method is capable of preventing the generation of peculiar patterns when increasing the number of gradation spatially, but may deteriorate the luminance efficiency and cause false contour problems.
Meanwhile, there is another conventional method of improving the gradation of an image, disclosed in U.S. Pat. No. 4,921,334 entitled “Matrix Liquid Crystal Display with Extended Gray Scale”. The disclosed method produces new medium gradations by switching neighboring liquid crystal driving voltages. However, this method is disadvantageous in that the number of gradations cannot be increased more than two times.
There is also a conventional spatial-temporal dithering method of improving the gradation of an image, disclosed in Three-Five systems (SID 2000 Seminar lecture notes, volume 1, M-13). This method combines a spatial dithering method using a 2×2 pixel mask, and a temporal dithering method using two different voltage levels adjacent to two sub-fields, and produces three additional gradations. This method is advantageous in that a lot of new gradations can be produced, but the resolution of an output image may deteriorate due to the use of a spatial dithering method. Also, this method generates the aforementioned peculiar patterns at the edge of an image or color-flattened region, and further requires additional circuits to perform this method.
In the event that a difference between the luminance levels of gradations is irregular, it is difficult to make the irregular difference regular by the above-mentioned conventional methods of improving the gradation of an image. As a result, the number of the gradations may decrease more and more.